mcd is millicandela, or 1/1000th of a candela.
µcd is microcandela, or 1 millionth of a candela, which as far as I know is never used since there's no way eyes can detect a change in intensity that tiny.
1 candela is one lumen projected into an angle of one steradian. To give some perspective, if a point-source like a light bulb is projecting 1256 lumens in all directions (say a 75 lamp), then the intensity is going to be 1256/4pi, or 100 candela in all directions. If you were to concentrate that amount of light into a tight spotbeam, the candela figure increases, the lumen figure stays the same.
